Method for manufacturing rack bar

ABSTRACT

A method for manufacturing a rack bar includes joining axial end portions of first and second bar members to each other, and forming a power transmission section on the second bar member. The first bar member has a toothed portion. When the second bar member is hollow, the method may further include thickening a wall of a portion of the second bar member along the axial direction so as to be coaxial with the first bar member. When the second bar member has a greater diameter than the first bar member, the method may further include cutting an outside diameter of a portion of the second bar member in the axial direction so as to be coaxial with the first bar member. The power transmission section is formed on the portion of the second bar member where the wall has been thickened and/or the outside diameter has been cut.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a rack bar.

BACKGROUND ART

A dual pinion type rack bar is a type of a rack bar for use in asteering apparatus of a vehicle, such as an automobile, having toothedportions at two locations, each toothed portion having a plurality ofrack teeth. One of the toothed portions is engaged with a steeringpinion of a steering shaft of a vehicle, and the other toothed portionis engaged with an assist pinion of an assisting mechanism. Some dualpinion type rack bars are designed such that the toothed portions at thetwo locations have an angular difference around the axis, depending on apositional relationship between the steering shaft and the assistingmechanism of the vehicle.

JP2014-124767A discloses a method for manufacturing a dual pinion typerack bar in which a first bar member having a toothed portion and asecond bar member arranged coaxially with the first bar member arefriction-welded by being rotated relative to each other around thecenter axis and then another toothed portion is formed on the second barmember by cutting. By forming the toothed portion on the second barmember after joining the first bar member and the second bar member toeach other, the accuracy of the angular difference between the toothedportion of the first bar member and the toothed portion of the secondbar member is improved.

In this method for manufacturing the rack bar, when there is amisalignment or inclination of the second bar member with respect to thefirst bar member to which the second bar member has been joined, thestraightness of the rack bar is degraded.

SUMMARY

Illustrative aspects of the present invention improve a shape accuracyof a rack bar formed by joining a second bar member to a first barmember formed with a toothed portion having a plurality of rack teeth.

According to an illustrative aspect of the present invention, a methodfor manufacturing a rack bar is provided. The method includes joining anaxial end portion of a first bar member and an axial end portion of asecond bar member to each other, the first bar member and the second barmember being hollow members, and the first bar member having a toothedportion along an axial direction, and the toothed portion having aplurality of rack teeth, thickening a wall of at least a portion of thesecond bar member, that has been joined to the first bar member, alongthe axial direction such that the portion of the second bar member iscoaxial with the first bar member, and forming a power transmissionsection on the wall-thickened portion of the second bar member.

According to another illustrative aspect of the present invention,another method for manufacturing a rack bar is provided. The methodincludes joining an axial end portion of the first bar member and anaxial end portion of a second bar member to each other, the first barmember having a toothed portion along an axial direction, the toothedportion having a plurality of rack teeth, and the second bar memberhaving a greater diameter than the first bar member, cutting an outsidediameter of at least a portion of the second bar member, that has beenjoined to the first bar member, in the axial direction such that portionof the second bar member is coaxial with the first bar member, andforming a power transmission section on the portion of the second barmember where the outside diameter has been cut.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view of an example of a steering apparatus accordingto an embodiment of the present invention.

FIG. 2 a front view of a rack bar installed in the steering apparatus.

FIG. 3 is a cross-sectional view of a rack bar according to a firstembodiment of the present invention.

FIG. 4 is a schematic view illustrating a step for manufacturing therack bar.

FIG. 5 is a schematic view illustrating another step for manufacturingthe rack bar.

FIG. 6 is a schematic view illustrating another step for manufacturingthe rack bar.

FIG. 7 is a schematic view illustrating another step for manufacturingthe rack bar.

FIG. 8 is a cross-sectional view of a rack bar according to a secondembodiment of the present invention.

FIG. 9 is a schematic view illustrating a step for manufacturing therack bar of FIG. 8.

FIG. 10 is a schematic view illustrating another step for manufacturingthe rack bar of FIG. 8.

FIG. 11 is a schematic view illustrating another step for manufacturingthe rack bar of FIG. 8.

FIG. 12 is a schematic view illustrating yet another step formanufacturing the rack bar of FIG. 8.

FIG. 13 is a schematic view illustrating an example of a deformation ofthe rack bar.

FIG. 14 is a cross-sectional view of a modified example of the rack baraccording to the second embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates an example of a steering apparatus 1 according to anembodiment of the present invention.

The steering apparatus 1 has a rack housing 2 and a rack bar 10 that ishoused in the rack housing 2 so as to be slidable in an axial direction.

A tie rod 3 is connected to each end of the rack bar 10 via a joint. Thewheels of a vehicle are turned by the movement of the rack bar 10 viathe tie rods 3 and a steering mechanism to which the tie rods 3 areconnected.

A steering gear box 4 is provided at one axial end portion of the rackhousing 2. A steering pinion (not shown) formed on an input shaft 5 tobe connected to a steering shaft is housed in the steering gear box 4.An assist gear box 6 is provided at the other axial end of the rackhousing 2. An assist pinion (not shown) to be driven by the motor 7 ofan assisting mechanism is housed in the assist gear box 6.

A first toothed portion 20 having a plurality of rack teeth to beengaging with the steering pinion and a second toothed portion 21 havinga plurality of rack teeth engaged with the assist pinion are formed onthe rack bar 10.

The steering pinion of the input shaft 5 is rotated by the rotatingoperation of the steering wheel, whereby the rack bar 10 engaged withthe steering pinion at the first toothed portion 20 is moved in theaxial direction. The driving force of the motor 7 of the assistingmechanism to be controlled depending on the steering force of thesteering wheel is transmitted to the rack bar 10 via the assist pinionengaged with the second toothed portion 21, whereby the movement of therack bar 10 by the rotating operation of the steering wheel is assisted.

FIG. 2 illustrates a configuration of the rack bar 10.

The rack bar 10 has a first bar member 11 on which the first toothedportion 20 having the plurality of rack teeth engaged with the steeringpinion and a second bar member 12 on which the second toothed portion 21having the plurality of rack teeth engaged with the assist pinion, andan axial end of the first bar member 11 and an axial end of the secondbar member 12 are joined to each other.

In the illustrated example, the first bar member 11 is formed as ahollow member being circular in cross section and made of a metalmaterial, such as carbon steel JIS-S45C. The rack teeth of the firsttoothed portion 20 of the first bar member 11 formed as a hollow memberare formed as described below, for example.

First, a flat teeth forming surface is on a portion of the hollow memberin the longitudinal direction where a toothed portion is to be provided(hereafter, teeth forming portion). The teeth forming surface is formedby, for example, deforming the teeth forming portion of the hollowmember using a press-forming die.

Next, the hollow member is placed in the forming die including a teethforming die to be pressed against the teeth forming surface andsurrounding the entire teeth forming portion of the hollow member, and amandrel is inserted into the hollow member. The material of the hollowmember at the teeth forming surface is plastically deformed by theinserted mandrel from the inside and is pushed into the teeth formingdie that is pressed against the teeth forming surface. This plasticworking is repeated with the mandrel to be inserted being exchanged tohave a gradually larger size, whereby a plurality of rack teethcorresponding to the teeth forming die is formed on the hollow member.

FIG. 3 illustrates a cross section of the rack bar 10 according to afirst embodiment of the present invention. In the first embodiment, thesecond bar member 12 is also formed as a hollow member being circular incross section and made of a metal material, such as carbon steelJIS-S45C. The rack teeth of the second toothed portion 21 of the secondbar member 12 are formed as described later in detail, for example, bycutting after the second bar member 12 is joined to the first bar member11.

The profile of the rack teeth of the first toothed portion 20 of thefirst bar member 11 may be the same as or different from the profile ofthe rack teeth of the second toothed portion 21 of the second bar member12, and a constant gear ratio (CGR) and a variable gear ratio (VGR) maybe combined.

FIGS. 4 to 7 illustrate steps for manufacturing the rack bar 10according to the first embodiment.

As shown in FIG. 4, the first bar member 11 on which the first toothedportion 20 has already been formed and the blank second bar member 12are coaxially arranged. In the illustrated example, the outside diameterof the blank second bar member 12 is greater than the outside diameterof the first bar member 11. However, the outside diameter of the blanksecond bar member 12 may be the same as the outside diameter of thefirst bar member 11, or may be smaller than the outside diameter of thefirst bar member 11. In the illustrated example, the inside diameter ofthe blank second bar member 12 is the same as the inside diameter of thefirst bar member 11. However, the inside diameter of the blank secondbar member 12 may be different from the inside diameter of the first barmember 11.

In the illustrated example, the axial end portion 12 a (joining endportion) of the second bar member 12 on the side of the first bar member11 is preformed, for example, by cutting, in a ring shape having thesame inside diameter and the same outside diameter as the axial endportion 11 a (joining end portion) of the first bar member 11 opposed tothe joining end portion 12 a. When the outside diameter of the blanksecond bar member 12 is smaller than the outside diameter of the firstbar member 11, upsetting may be performed in advance on the joining endportion 12 a of the second bar member 12 so that the outside diameter ofthe joining end portion 12 a is made equal to or greater than theoutside diameter of the joining end portion 11 a. When the insidediameter of the blank second bar member 12 is greater than the insidediameter of the first bar member 11, the joining end portion 12 a of thesecond bar member 12 may be swaged in advance so that the insidediameter of the joining end portion 12 a is made equal to or smallerthan the inside diameter of the joining end portion 11 a.

As shown in FIG. 5, the second bar member 12 is moved toward the firstbar member 11, and the respective end faces of the joining end portion11 a of the first bar member 11 and the joining end portion 12 a of thesecond bar member 12 are abutted against each other. The first barmember 11 is then rotated around its center axis.

The metal structures of the joining end portion 11 a and the joining endportion 12 a are changed by the friction heat caused by the relativerotation of the respective end faces of the joining end portion 11 a ofthe first bar member 11 and the joining end portion 12 a of the secondbar member 12 abutted against each other, and pressure is applied to theend faces, whereby the joining end portion 11 a and the joining endportion 12 a are pressure-welded to each other.

Due to the joining end portion 12 a being preformed in a ring shapehaving the same inside diameter and the same outside diameter as thejoining end portion 11 a, the plastic flow amount of the joining endportion 11 a and the plastic flow amount of the joining end portion 12 aaround the pressure-welding faces thereof become substantially equal toeach other during the friction welding of the joining end portion 11 aof the first bar member 11 and the joining end portion 12 a of thesecond bar member 12, whereby the joint between the joining end portion11 a and the joining end portion 12 a becomes more secure.

In the state in which the first bar member 11 and the second bar member12 are joined to each other, misalignment or inclination of the secondbar member 12 with respect to the first bar member 11 may occur in somecases, for example, due to assembling errors caused by manufacturingapparatuses and the pressure distribution at the pressure-welded facesduring the friction welding.

Hence, as shown in FIG. 6, the second bar member 12 is corrected so asto be coaxial with the first bar member 11. This correction of thesecond bar member 12 includes wall-thickening of the second bar member12. In the illustrated example, the wall of the blank second bar member12 having a greater outside diameter than the first bar member 11 isthickened by reducing the diameter of the blank second bar member 12with its axial length being substantially unchanged such that the secondbar member 12 has the same outside diameter as the first bar member 11.This wall-thickening of the second bar member 12 may be performed by,for example, swaging the second bar member 12 using a plurality of diesrotating around the circumference of the second bar member 12 andstriking the second bar member 12 in the radial direction to change thecross-sectional shape of the second bar member 12. When the outsidediameter of the blank second bar member 12 is equal to or smaller thanthe outside diameter of the first bar member 11, the wall of the secondbar member 12 may be thickened by making its axial length shorter. Thewall of the second bar member 12 may be thickened such that the outsidediameter of the second bar member 12 is different from the outsidediameter of the first bar member 11. In the illustrated example, thewall of the second bar member 12 is thickened along the entire axiallength of the portion of the second bar member 12 other than the joiningend portion 12 a. The wall of the second bar member 12 may thickenedonly along a portion of the second bar member 12 along the axialdirection including a section where the second toothed portion 21 is tobe formed in a later process.

Then, as shown in FIG. 7, rack teeth are formed on the portion of thesecond bar member 12 to form the second toothed portion 21 by cuttingusing a broaching machine or the like, and heat treatment, such asquenching, may be optionally performed on the second toothed portion 21.Surface finishing, such as grinding, may be performed on the second barmember 12 between the wall-thickening and the teeth cutting.

The rack bar 10 manufactured as described above can be reduced in weightbecause the first bar member 11 and the second bar member 12 are bothhollow.

The wall thickness of the predetermined portion of the second bar member12 on which the second toothed portion 21 is formed can be obtainedsecurely by increasing the wall thickness of the second bar member 12,whereby the degree of freedom of the shape of the rack teeth of thesecond toothed portion 21 can be enhanced. Still further, the second barmember 12 can be corrected so as to be coaxial with the first bar member11 and the straightness of the rack bar 10 can also be enhanced byperforming the wall-thickening for the second bar member 12.

FIG. 8 illustrates a cross section of a rack bar 10 according to asecond embodiment of the present invention.

The second bar member 12 of the rack bar 10 according to the secondembodiment is formed as a solid member being circular in cross sectionand made of a metal material, such as carbon steel JIS-S45C. The rackteeth of the second toothed portion 21 of the second bar member 12formed as a solid member is formed by cutting, for example.

The first bar member 11 may also be formed as a solid member, and therack teeth on the first toothed portion 20 of the first bar member 11formed as a solid member are formed by cutting, for example.

FIGS. 9 to 12 illustrate steps for manufacturing the rack bar 10according to the second embodiment.

As shown in FIG. 9, the first bar member 11 on which the first toothedportion 20 has already been formed and the blank second bar member 12are arranged so as to be coaxial with each other. The outside diameterof the blank second bar member 12 is greater than the outside diameterof the first bar member 11.

In the illustrated example, the axial end portion 12 a (joining endportion) of the second bar member 12 on the side of the first bar member11 is preformed, for example, by cutting, in a ring shape having a sameshape as, i.e., having the same inside diameter and the same outsidediameter as the axial end portion 11 a (joining end portion) of thefirst bar member 11 opposed to the joining end portion 12 a. When thefirst bar member 11 is a solid member, the joining end portion 12 a ofthe second bar member 12 is preformed in a column shape having a sameshape as, i.e., having the same outside diameter as the joining endportion 11 a.

As shown in FIG. 10, the second bar member 12 is moved toward the firstbar member 11, and the respective end faces of the joining end portion11 a of the first bar member 11 and the joining end portion 12 a of thesecond bar member 12 are abutted against each other. The first barmember 11 is then rotated around its center axis.

The metal structures of the joining end portion 11 a and the joining endportion 12 a are changed by the friction heat caused by the relativerotation of the respective end faces of the joining end portion 11 a ofthe first bar member 11 and the joining end portion 12 a of the secondbar member 12 abutted against each other, and pressure is applied to theend faces, whereby the joining end portion 11 a and the joining endportion 12 a are pressure-welded to each other.

Due to the joining end portion 12 a being preformed in a same shape asthe joining end portion 11 a, the plastic flow amount of the joining endportion 11 a and the plastic flow amount of the joining end portion 12 aaround the pressure-welding faces thereof become substantially equal toeach other during the friction welding of the joining end portion 11 aof the first bar member 11 and the joining end portion 12 a of thesecond bar member 12, whereby the joint between the joining end portion11 a and the joining end portion 12 a becomes more secure.

As shown in FIG. 11, in the second embodiment, the second bar member 12is corrected so as to be coaxial with the first bar member 11 byouter-diameter cutting. In the illustrated example, the outside-diametercutting is performed along the entire axial length of the second barmember 12 such that the second bar member 12 is coaxial with the firstbar member 11 and has the same outside diameter as the first bar member11 along the entire axial length of the second bar member 12. However,the second bar member 12 may be formed to have an outside diameterdifferent from the outside diameter of the first bar member 11. Theoutside-diameter cutting may be performed only along a portion of thesecond bar member 12 in the axial direction including a section wherethe second toothed portion 21 is formed in a later process so that onlythe portion of the second bar member 12, including the section where thesecond toothed portion 21 is formed, is formed is be coaxial with thefirst bar member 11.

Then, as shown in FIG. 12, rack teeth are formed on the given section ofthe second bar member 12 to form the second toothed portion 21 bycutting using a broaching machine or the like, and heat treatment, suchas quenching, may be optionally performed on the second toothed portion21. Surface finishing, such as grinding, may be performed on the secondbar member 12 between the outside-diameter cutting and the teethcutting.

The rack bar 10 manufactured as described above has improvedstraightness because the second bar member 12 is formed to be coaxialwith the first bar member 11 by the outside-diameter cutting as shown inFIG. 11.

The outside-diameter cutting shown in FIG. 11 also removes the internalstress remaining in the surface layer of the blank second bar member 12.

The solid member serving as the blank second bar member 12 is generallyproduced by a drawing process. This type of solid member has internaltensile stress remaining in its surface layer and internal compressivestress remaining in its deep layer. The internal tensile stress isgreater at a location closer to the surface layer, and the internalcompressive stress is greater at a location closer to the deep layer.The internal tensile stress or the internal compressive stress remainingin the intermediate layer between the surface layer and the deep layeris smaller than the internal stress remaining in the surface layer andthe internal stress remaining the deep layer.

By cutting the outside diameter of the second bar member 12, the surfacelayer of the material is removed, and the intermediate layer havingsmaller internal stress than the surface layer is exposed. The rackteeth of the second toothed portion 21 are formed on this intermediatelayer. Therefore, even when the internal stress is released by teethcutting and heat treatment, the deformation of the second bar member 12due to the release of the internal stress is suppressed.

As the deformation of the second bar member 12 due to the release of theinternal stress, torsion around the center axis thereof can be taken asan example as shown in FIG. 13. In the case that the torsion of thesecond bar member 12 occurs, the pitch of the rack teeth on the secondtoothed portion 21 is changed, the engagement of the rack teeth with thepinion is degraded and there is a danger that transmission efficiencymay be degraded. Removing the internal stress remaining in the surfacelayer of the blank second bar member 12 by performing theoutside-diameter cutting is particularly effective in suppressing thetorsion of the second bar member 12 which may cause degradation intransmission efficiency.

The cutting stock of the blank second bar member 12 for theoutside-diameter cutting is set in consideration of, for example, apossible misalignment or inclination of the second bar member 12 withrespect to the first bar member 11 and a distribution of internal stressremaining in the blank second bar member 12. An example of preferablecutting stock is 1 mm to 2 mm in diameter.

As described above, the second bar member 12 joined to the first barmember 11 is formed so as to be coaxial with the first bar member 11 byperforming the outside-diameter cutting, and then the rack teeth of thesecond toothed portion 21 are formed at the portion of the second barmember 12 subjected to the outside-diameter cutting, whereby thestraightness of the rack bar 10 can be enhanced, the deformation of thesecond bar member 12 can be suppressed, and the shape accuracy of therack bar 10 can be enhanced.

While the second bar member 12 has been described as a solid memberhaving a greater diameter than the first bar member 11 in the exampledescribed above, the second bar member 12 may be a hollow member havinga greater diameter than the first bar member 11 as shown in FIG. 14.

Also with the hollow second bar member 12, a rack bar can bemanufactured by cutting the outside diameter of at least a portion ofthe second bar member 12 that has been joined to the first bar member 11to form the portion of the second bar member 12 to be coaxial with thefirst bar member 11 and by forming a power transmission sectionincluding rack teeth, a screw groove or the like on the portion of thesecond bar member 12 that has been subjected to the outside-diametercutting.

The manufacturing method according to the first embodiment may include,between the wall-thickening process shown in FIG. 6 and the teethcutting process shown in FIG. 7, cutting of the outside diameter of thesecond bar member 12 with the center axis of the first bar member 11being a reference. This additional step further improves thestraightness of the rack bar 10, and removes the internal stressremaining in the surface layer of the blank second bar member 12.

A hollow member is also generally produced by a drawing process.Therefore, the hollow member also has internal tensile stress remainingin its surface layer and internal compressive stress remaining in itsdeep layer, and the tensile or internal compressive stress remaining inthe intermediate layer between the surface layer and the deep layer issmaller than the internal stress remaining in the surface layer and theinternal stresses remaining in the deep layer.

While the power transmission section formed on the second bar member 12has been described as the second toothed portion 21 having a pluralityof rack teeth in the example described above, the power transmissionsection is not limited to such a toothed portion, and may be the screwgroove of a ball screw.

This application is based on Japanese Patent Application Nos.2015-217043 and 2015-217044, both filed on Nov. 4, 2015, the entirecontents of which are incorporated herein by reference.

1-6. (canceled)
 7. A method for manufacturing a rack bar, the methodcomprising: joining an axial end portion of the first bar member and anaxial end portion of a second bar member to each other, the first barmember having a toothed portion along an axial direction, the toothedportion having a plurality of rack teeth, and the second bar memberhaving a greater diameter than the first bar member, cutting an outsidediameter of at least a portion of the second bar member, that has beenjoined to the first bar member, in the axial direction such that portionof the second bar member is coaxial with the first bar member, andforming a power transmission section on the portion of the second barmember where the outside diameter has been cut.
 8. The method accordingto claim 7, further comprising performing a heat-treatment on the powertransmission section formed on the second bar member.
 9. The methodaccording to claim 7, wherein the first bar member and the second barmember are joined to each other by a friction welding, the frictionwelding including rotating the first bar member and the second barmember relative to each other around a center axis of the first barmember.
 10. The method according to claim 9, wherein the first barmember and the second bar member are joined to each other by thefriction welding in a state in which the axial end portion of the secondbar member is preformed to have a same shape as the axial end portion ofthe first bar member.